This invention relates to a method of controlling a long-stroke, hydraulic operating cylinder, in particular a screw-down cylinder for an edging stand, wherein the piston, to which a load can be applied at both ends, and which can be locked hydraulically along its stroke in various working positions, is abruptly loaded with an external force after it has been locked.
Long-stroke hydraulic cylinders, as they are in particular used for adjusting the roll gap of edging stands, are locked in the respective working position chosen along the piston stroke. When the pistons are abruptly subjected to a load, for instance due to an increase in the rolling force immediately after the initial pass, a piston displacement occurs despite such locking due to the compressibility of the hydraulic medium and the elastic behavior of the construction parts carrying the hydraulic medium. A control provided for actuating the respective working position and tracking the pistons under changing conditions can only operate with its own control delay, so that in the case of an abrupt load acting on the pistons a short-time yielding of the operating cylinder must be expected, which leads to difficulties in the case of higher demands as to the accuracy of the maintenance of the respective working position.
It is therefore the object underlying the invention to provide a method of controlling a long-stroke, hydraulic operating cylinder, in particular a screw-down cylinder for an edging stand as described above such that the displacement of the hydraulically locked piston can be restricted to a comparatively small degree in the case of an abrupt load acting on the piston.
This object is solved by the invention in that the piston is locked under a hydraulic pressure chosen in dependence on the respective working position, which hydraulic pressure decreases along the stroke against the direction of action of the external force.
The invention is based on the knowledge that in a working position of the piston with a comparatively small cylinder volume, the pressure of the hydraulic medium at the end of the piston exposed to a relief in the case of an external impact load is decreased much faster at this end during a piston displacement due to the comparatively low volume than the pressure of the hydraulic medium is increased at the opposite end of the piston in the vicinity of the larger volume of hydraulic medium. When the external impact load of the piston occurs, however, in a working position with a comparatively large cylinder volume at the end of the piston relieved during an impact load, there is a faster increase in pressure in the vicinity of the comparatively low volume of the hydraulic medium at the opposite end of the piston, whereas the pressure of the hydraulic medium in the vicinity of the larger cylinder volume at the other end of the piston is decreased at a correspondingly lower rate. In the case of a working position with a piston displaced towards the end of the stroke against the direction of impact, this means a comparatively large piston displacement during the impact load, whereas this impact-related displacement at the opposite end of the stroke remains comparatively small due to the quickly increasing counterpressure.
In order to limit the impact-related piston displacement, a corresponding counterpressure must be ensured by means of the hydraulic medium without having to fear an overload of the cylinder due to the impact-related increase in pressure. From these requirements it follows that to limit the impact-related piston displacements, the pressure of the hydraulic medium, at which the piston is locked in the respective working position, should decrease along the stroke against the direction of action of the external force acting on the piston. The piston can thus be locked in the end portion of the stroke facing the impact load with the maximum pressure of the hydraulic medium, because the increase in pressure occurring during an external impact load remains limited. Due to the high increase in pressure in the case of an abrupt load acting on the piston which is locked in the vicinity of the opposite stroke end, the locking pressure must be chosen correspondingly low, so as not to endanger the cylinder at the one end of the piston due to the increase in the pressure of the hydraulic medium caused by the influence of the external force. The piston displacement nevertheless remains small because of the quickly increasing counterforce. Since the piston is locked under a locking pressure that depends on the respective working position of the piston along its stroke, the impact-related piston displacement can thus be restricted to a degree that is admissible for most applications without any additional measures.
To be able to lock the piston in the respective working position under a hydraulic pressure provided for this position, the piston can already be subjected to the locking pressure provided for this working position when it approaches such working position, so that in cooperation with the position control, a corresponding counterpressure must build up, when the piston should firmly be held in the respective working position.